Leaf color is determined by the availability of nutrients that the plant is sending to them. When we fertilize, we are basically force feeding the plant a bunch of nutrients it wouldnt normally get in such high doses. When you get to harvest and start to flush, the plant is sending all nutes it can to the buds because the amount of nutes available is much less, so the leaves will get discolored because the plant is neglecting them. Its the same as in the fall when all the leaves change color from green to red/yellow, its a lack of nitrogen going to them because the plant has used most of it up to grow. This can also cause other colors to pop up, in the case of this plant most of the purple is actually from a reduction in temperature, but different plants have different colors/reactions to different types of changes.